I realize this is a late contribution to the conversation, however . . .

As far as a definition goes, I have lately relied on a paraphrase of some terminology that Chris Welty had in a 2003 publication - in essence it amounts to an Ontology (as a product, rather than as a pursuit) is "an artifact that represents some part of the world".

This is helpful in several ways, it is simple to understand; it does not tie itself to any confusing presupposed ideas about concepts, entities, relations, etc; and it shows that current solutions (RDF? OWL? KIF?) are not the only thing that matters. I use this definition when presenting Leo's Ontology Spectrum, to show that a number of different approaches (based on the need for depth of meaning in the artifact) can all be "ontologies" - and most of all, that an Ontology is not definitionally an OWL document.